Posted
by
ScuttleMonkey
on Wednesday June 13, 2007 @12:08PM
from the celestial-slingshot dept.

mcgrew writes to mention New Scientist is reporting that scientists have clocked matter traveling at 99.999% the speed of light. "The fastest flows of matter in the universe shoot out of dying stars at more than 99.999% the speed of light, new observations reveal. When a massive star runs out of fuel, it collapses to form a black hole or a neutron star. In the process, some of the matter from the star also explodes outward at blistering speeds, producing an intense burst of gamma rays and other radiation."

The "speed of force" as you put it, is not really a speed inherent to force. You would be measuring how fast a tensile or compressive wave passes through the pole, same as the speed of sound through it. It would be much slower than the speed of light.

I asked this question in a physics class and the answer I got, which makes quite a bit of sense, is that force travels through a material at the speed of sound. So if in your example your 500 foot pole was made of steel, the opposite end starts moving roughly 30 milliseconds after you push the near end. (The speed of sound in steel is very roughly 5000 meters/sec.)

Wow, I was hoping that there would already be an explanation answering this, but here you go: The speed of "force" as you put it, is actually quite slow. It's a actually the speed of sound through the object. Why? Because when you push the rod, you're bumping the molecules, they have to push the molecules in front of them, and on until you reach the end. This is actually a sound wave propagating the medium, you just usually can't hear it. Now, if you had a perfectly rigid pole (cue penis jokes here) it would seeming move instantly. However, no known substance is anywhere close to perfectly rigid. Even atomic nuclei, which are, far, far more rigid than bulk matter, behave like drops of fluid and can have waves propagate through them. So no, you can't forge a pole to another planet and communicate instantly, it would be hugely slower than normal radio.

I know you're joking, but even a perfectly rigid pole would be subject to the propagation of forces. Think about what forces have to propagate in order to tell the other end of the pole to move. One atom has to repel the next atom using electromagnetic force, weak and strong nuclear forces, which has to in turn repel the next atom, etc, etc. There is an elastic repulsive process which goes all the way down the pole until it reaches the other end. And we know the fastest that this can happen is the speed of light. So the pole will be momentarily compressed as the force propagates.

No information can travel faster than the speed of light, as a general rule.

Actually, and believe you me I am no damn physicst (can't even spell it), a photon has no "resting mass", but does have momentum, which implies that there is an upper limit to is mass which cannot be zero.

The steel pole isn't going to have a constant compression rate. As the pole is compressed more and more its density increases thus changing the speed of sound through it. Both ends of the pole would likely "catch up" with each other before we experienced any major space-time paradoxes that destroyed the universe as we know it.

What happens is that velocities don't add together using the simple addition rule. See the Wiki [wikipedia.org]. Once you're at reasonable fractions of the speed of light (say about 10%; certainly by the time you hit 50%), the fact that it's not simple addition makes an appreciable difference. In your example, each space-ship measures the other as going at about 94% of the speed of light.

What really happens is that velocities don't add like that. They seem to for everyday objects, but relativistic effects become important at 0.7c. You should add them according to the Einstein formula:
v = (B+v')/(1 + Bv')
where B is the speed of one ship relative to an observer at rest (0.7c), and v' is the speed of the other ship in it's frame (0.7c). So the speed of one ship relative to the other is just v = 1.4/1.49 = 0.94c.
You can see that, for small speeds, the product in the denominator is small, so we have the usual addition.

Nope.
Time dilation and space contraction take place here. Relativity states that if, say, you were going at 75% of the speed of light, and shot a missile at 50% the speed of light, neither you, nor the torpedo, nor a 3rd observer would see the torpedo go faster than light. They'd see it go juuust under c, about 95% of c. In relativity, adding of velocities isn't as simple as absolute v + relative v, it's an asymptotic function that means you never actually reach the speed of light.

We think it's time goes slow, it thinks our time goes slow. It's one of the symmetries of a Lorentz transformation. What happens is that when one of the observers accelerates so that it can sit down and compare notes with the other observer the observer that did the accelerating will have seen less time go by. It's a peculiarity of the geometry of spacetime that an inertial observer takes the path of longest proper time, that is the time that the observer will see go by.

For example, those little thingies with the black and white paddles in them that look like light bulbs from middle school science class work on the idea that photons transfer and take momentum from stuff they interact with. Momentum is a quality very closely tied with mass.

Crookes radiometer (the aforementioned little thingy with the black and white paddles) does not rotate due to light imparted momentum (the force is too small). This theory of the rotation is disproved by the fact that after a certain point making the vacuum in the bulb stronger reduces the effect, which is the opposite of the expected result if the rotation was due to radiation force.

The actual forces responsible for rotation are a combination of forces due to molecule movement between the hot and cold sides of the vanes near the edges. Wikipedia has a good write up about it here [wikipedia.org].

There is an invariant mass for an object, i.e. a quantity that remains the same in all reference frames. This can be calculated based on energy and momentum. True of photons as well. Photons don't have a rest mass because rest mass is defined as the mass of an isolated and at rest relative to the observer object. Photons can't be at rest relative to an observer (and if they are isolated they are travelling at c).

What really happens is that velocities don't add like that. They seem to for everyday objects, but relativistic effects become important at 0.7c.

Your post is right on. I might add that when relativistic effects become important for everyday objects might be a matter of application. For example, some GPS systems need to account for relativistic effects for the relativive motion of objects in orbit with respect to the surface of the earth (moving much smaller than 0.7c). It depends on the accuracy required.